Ceramic honeycomb structures consisting of a plurality of cells or passages separated by thin walls in parallel relationship to the longitudinal axis of the structures with, in some instances, discontinuities extending transversely through those walls are well known to the art. Such units have seen extensive service as filters for fluids, in heat exchangers, as substrates in various chemical processes, and, more recently, the walls of those bodies have been coated with a catalyst capable of transforming noxious fumes from the discharge gases emanating from internal combustion engines and wood-burning stoves into innocuous components. Those monolithic structures have customarily been fabricated via extrusion.
U.S. Pat. No. 4,235,583 is directed to forming extrusion dies for use in making such honeycomb structures. As was explained therein, the walls of the dies, conventionally prepared from steel, were worn away rather rapidly due to the high pressures encountered in the extrusion process and abrasion from the ceramic particles being extruded. As a result it was commonplace for the die to be replaced after a relatively short service life because the slots therein had become so worn that the wall sections of the ceramic monolith produced were thicker than desired. The method of the patent accomplished three main objectives: it provided means for significantly extending the service life of the dies; it enabled the slot dimensions to be reconstructed to permit the dies to be reused; and it allowed the cutting of the dies to be undertaken utilizing forming techniques requiring less criticality in control and of less expense.
The crux of the patented method lay in cutting the slots in the extrusion or forming end of the die larger than the desired wall thickness of the passages in the monoliths, and thereafter electroless plating the slot-defining surfaces of the die with nickel, chromium, cobalt, or cobalt-nickel to obtain a slot width corresponding to the desired wall thickness. The resultant coating not only provides a harder, more wear-resistant surface than the steel substrate, thereby permitting longer extrusion campaigns, but also, where the coating reaches the point of excessive wear, it may be stripped away from the die surface and the die replated to its original slot width. This latter feature makes possible repeated use of the die.
The capability of building up the slots through electroless plating served two valuable and related purposes. First, it enabled the cutting of the die to be undertaken by means of less expensive, less sophisticated forming techniques. For example, a whole gamut of operable cutting techniques can be considered including electrical discharge machining, wire cutting, milling, wheel grinding, laser cutting, and electron beam cutting. Second, it permitted the production of very narrow slots which could only be machined to specification with great difficulty and control.
Whereas those coatings resulting from electroless plating of the basic steel dies did, indeed, markedly increase the service life of the dies, harder and more wear-resistant coatings have been sought to make possible longer extrusion campaigns before replacement of the coating becomes necessary. Ceramic honeycomb monoliths vary widely in structure. For example, for use in the exhaust stacks of electric power generating stations, catalyst-coated monoliths having as few as 5 parallel passages per square inch with wall thicknesses of about 0.060" and greater can be employed. The particulate material in the exhaust gases passes out through the passages while the catalyst acts upon the NO.sub.X and SO.sub.x components of the gases. In contrast, ceramic honeycomb monoliths are currently being utilized commercially in catalytic conversion units for automobiles which have 400 parallel passages per square inch with consequent wall thicknesses of less than about 0.008", and monoliths having as many as 1500 passages per square inch and wall thicknesses of about 0.003" for use in heat regenerator units have been predicted. Therefore, a deposition technique providing greater control than is possible with electroless metal plating in building up such coatings was demanded in order to insure coatings of precise uniformity in thickness.
Accordingly, the principal objective of the present invention is to produce an extrusion die for forming ceramic honeycomb monoliths wherein the slot-defining surfaces of the die are covered with a coating that is much harder and more wear-resistant than can be achieved through electroless metal plating of nickel, chromium, cobalt, cobalt-nickel, or nickel with abrasion-resistant particles such as SiC and Al.sub.2 O.sub.3, and wherein the method of applying that coating makes possible greater uniformity in the thickness of the coating than can be accomplished by means of electroless plating.
A specific objective of the present invention is to provide such an extrusion die for forming ceramic honeycomb monoliths having at least 400 parallel passages per square inch with wall thicknesses of less than about 0.008".